It is known in the art that automotive vehicles are commonly equipped with audio radios that receive and process signals relating to amplitude modulation/frequency modulation (AM/FM) antennas, satellite digital audio radio systems (SDARS) antennas, global positioning system (GPS) antennas, digital audio broadcast (DAB) antennas, dual-band personal communication systems digital/analog mobile phone service (PCS/AMPS) antennas, Remote Keyless Entry (RKE) antennas, Tire Pressure Monitoring System antennas, and other wireless systems.
Currently, it is known that patch antennas are employed for reception of GPS [i.e. right-hand-circular-polarization (RHCP) waves] and SDARS [i.e. left-hand-circular-polarization (LHCP) waves]. SDARS patch antennas may be considered to be a ‘single element’ antenna that incorporates performance characteristics of ‘dual element’ antennas that essentially receives terrestrial and satellite signals. SDARS—offer digital radio service covering a large geographic area, such as North America. Satellite-based digital audio radio services generally employ either geo-stationary orbit satellites or highly elliptical orbit satellites that receive uplinked programming, which, in turn, is re-broadcasted directly to digital radios in vehicles on the ground that subscribe to the service. SDARS also use terrestrial repeater networks via ground-based towers using different modulation and transmission techniques in urban areas to supplement the availability of satellite broadcasting service by terrestrially broadcasting the same information. The reception of signals from ground-based broadcast stations is termed as terrestrial coverage. Hence, an SDARS antenna is required to have satellite and terrestrial coverage with reception quality determined by the service providers, and each vehicle subscribing to the digital service generally includes a digital radio having a receiver and one or more antennas for receiving the digital broadcast. GPS antennas, on the other hand, have a broad hemispherical coverage with a maximum antenna gain at the zenith (i.e. hemispherical coverage includes signals from 0° elevation at the earth's surface to signals from 90° elevation up at the sky). Emergency systems that utilize GPS, such as OnStar™, tend to have more stringent antenna specifications as they also incorporate cellular phone communication antennas.
Unlike GPS antennas which track multiple satellites at a given time, SDARS patch antennas are operated at higher frequency bands and presently track only two satellites at a time. Thus, the mounting location for SDARS patch antennas makes antenna reception a sensitive issue with respect to the position of the antenna on the vehicle. As a result, SDARS patch antennas are typically mounted exterior to the vehicle, usually on the roof, or alternatively, inside the vehicle in a hidden location. Even further, although patch antenna circular polarization patterns are generally omni-directional in the azimuth plane so that reception does not favor any particular direction, antenna reception may be limited due to antenna position relative to the vehicle.
Having a null in a certain direction reduces the signal reception to a smaller spatial region. As a result, some portion of the antenna's reception becomes useless and thereby limits the functionality of the antenna. In one scenario, orientation of a patch antenna in a diversity application may lead to the deployment of additional antennas positioned throughout the vehicle to cover all directions of possible signal reception.
Thus, when mounted inside a vehicle, conventional patch antennas have inherent performance issues relating to directionality of the reception. Accordingly, it is therefore desirable to provide an antenna unit that improves the directionality of patch antenna gains at low elevation angles to improve the terrestrial reception, and, in a diversity application, reduce the number of patch antennas needed to compensate for nulls in the pattern.